1. Field of the Invention
The present invention relates to a hydraulic system for use in working vehicles such as agricultural tractors.
2. Description of the Prior Art
For the purpose of raising and lowering a working machine carried on such a vehicle, a conventional hydraulic system comprises, for example, as disclosed in Japanese Patent Publication No. 20841/73, a hydraulic cylinder operable to raise and lower the working machine; a hydraulic pump; an oil tank; a manual valve constructed to be shiftable to a stationary position in which a working oil discharged from the hydraulic pump is released into the oil tank, a raised position in which the working oil is supplied into the hydraulic cylinder, and a lowered position in which the working oil supplied into the hydraulic cylinder is passed back into the oil tank; and a check valve interposed in an oil passage between the manual valve and the hydraulic cylinder for permitting a flow of the working oil in a normal direction from the manual valve toward the hydraulic cylinder and blocking a flow of the working oil in the reverse direction. In such known hydraulic system, the working machine can be lowered by its own weight. For this reason, a by-pass detouring the check valve is connected to an oil passage between the hydraulic cylinder and the manual valve, so that the working oil from the hydraulic cylinder can be passed through the by-pass and the manual valve back into the oil tank. But, when the manual valve has been shifted to the stationary position to maintain the working machine in the current position, the working oil intended to flow from the hydraulic cylinder through the by-pass back into the oil tank must be blocked by the manual valve.
In general, however, the manual valve is not so excellent in oil passage-closing function as the check valve and hence, it is difficult to reliably maintain the now position of the working machine for a long time.
There is also known a hydraulic system for a working vehicle having two working machines carried, for example, on its front and rear portions, which comprises a pair of hydraulic cylinders; a switch valve for permitting an output of a manual valve to selectively communicate with either of the pair of hydraulic cylinders; a pressure control valve which is opened upon application of an overload to the hydraulic cylinder selectively put into communication by the switch valve, thereby escaping a working oil within that hydraulic cylinder into the oil tank; and a throttle valve for throttling an oil passage to limit the operational speed of the hydraulic cylinder selectively put into communication by the switch valve. In such hydraulic system, when the working vehicle is left to stand for a long time, it is necessary to previously prevent the issuing of the working oil from each of the hydraulic cylinders in order to maintain the working machine stationary in the current position. For this purpose, for example, Japanese Utility Model Publication No. 4403/83 teaches the use of a stop valve capable of timely closing an oil passage for every cylinder. With this construction, the stop valves are required as many as two and therefore, the number of valves used in the whole apparatus may be necessarily increased, resulting in a difficulty to avoid increases in size and cost.